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A new study finds that deep-space travel could warp reaction time.

The first astronauts who set foot on the moon were quarantined for three weeks when they returned to Earth. Scientists weren't sure what kinds of lunar germs they might have brought back with them.

That level of caution may sound absurd today, but a new study shows trips to outer space can still mess with astronauts on a physiological level.

New research from Johns Hopkins finds that long-term deep space missions can alter brain proteins and cause cognitive deficits like lapses in attention and slower reaction times. Researchers came to this conclusion by exposing rats to high-energy particles that simulate the conditions that astronauts would experience in deep space, then running them through a series of tests that mimic the fitness assessments that astronauts, pilots, and soldiers are required to take.

But the strange thing scientists found is that deep-space conditions don't affect everyone the same way. About half of the rats tested emerged entirely unaffected. The others began showing symptoms about seven weeks after exposure to space-like conditions. And once impairments appeared, they never went away. (Some rats showed improvement over time, however, raising the question of whether recovery is possible.)

The difference comes down to an individual's resilience after exposure to radiation. In space, astronauts who leave their space vehicles for space walks or other work are exposed to radiation from the sun's subatomic particles, solar flares, cosmic rays, etc. Even landing on the moon is a risk, since it doesn't have the kind of planetwide magnetic field that protects us on Earth. (Mars, too, is a higher radiation environment than back home.)

If the findings translate to humans, scientists believe they might be able to identify a biological marker that would help determine how an individual astronaut's brain might respond to a deep-space mission before she rockets into the stars.

The idea would be to help at-risk astronauts better protect themselves in space, says Catherine Davis, the study's lead author. “As with other areas of personalized medicine, we would seek to create individual treatment and prevention plans for astronauts we believe would be more susceptible to cognitive deficits from radiation exposure,” she said in a statement. That might mean wearing an additional radiation shield, or limiting the duration of space walks.

Scientists say the astronauts who are in space right now are less at-risk for the brain deficits revealed in the study because the International Space Station is close enough to the Earth's magnetic field that they're somewhat protected.

But identifying what makes someone more likely to be adversely affected by radiation exposure in space could help non-astronauts, too. People on Earth are routinely exposed to radiation—in some work environments and for some medical treatments, for instance—and understanding how radiation might affect someone before they’re exposed could help mitigate the associated risks.

Elsewhere, scientists are already exploring how deep-space missions can affect other parts of the body and whether trips to space may increase cancer risks.